Linear voltage regulator circuits are used to create a clean, well regulated output voltage from some higher, noisy voltage supply source. Such regulator circuits are needed in most electrical systems to provide clean voltage, such as for industrial/automotive circuit applications where the environment is particularly noisy, or such as for wireless applications where the battery power fluctuates and frame synchronization glitches would become very apparent in the audio band.
High performance linear regulator circuits generally have very high gain and need to be frequency compensated in order to have stable performance over a very wide range of operating conditions. The higher the performance and wider the conditions, then the harder it is to provide simple compensation schemes to keep the regulator stable. Conditions include a large range of dropout voltages (difference between input supply voltage Vin and regulated output voltage Vout), a large range of load currents, and a large variety of off-chip capacitors. There is also temperature variation and technology process uncertainty especially for the pass transistor which switches Vin to Vout. Various kinds of frequency compensation schemes are used to provide stability. Examples include Miller compensation, nested Miller loops, and slow-rolloff compensation, along with additional off-chip or off-die load capacitor that may be part of the compensation. It's hard to find simple, small, frequency compensation schemes, which are desirable for cost and compactness reasons; this minimal size preference place further restrictions on the compensation scheme.
FIG. 1A illustrates a prior art typical linear voltage regulator with its frequency compensation element 140, and C load, 150. The goal of the circuit is to monitor the output voltage Vout via feedback and comparing it to some constant valued reference voltage Vref. When Vout is too high or too low, the circuit will self-adjust so that Vout returns to its nominal value, so that Vout remains essentially constant. There are three stages, 110, 120, 130, partly for high gain (performance) purposes. There are several phase and gain shifts resulting from the various high impedance nodes and feedforward paths from the stages and the output objects. The compensation and load capacitors must be selected to avoid too much cumulative phase shift that would create positive feedback and make the circuit unstable. That is, the compensation must balance and locate the poles and zeroes at such frequencies so as to provide sufficient phase margin. High performance voltage regulators often require large or complicated compensation components to be stable. Furthermore, the traditional compensation elements interact with each other and are difficult to adjust independently, making it hard to provide optimal compensation.